Module Debug Info: Emit namespaced C++ forward decls in the correct module.

The AST merges NamespaceDecls, but for module debug info it is
important to put a namespace decl (or rather its children) into the
correct (sub-)module, so we need to use the parent module of the decl
that triggered this namespace to be serialized as a second key when
looking up DINamespace nodes.

rdar://problem/29339538

llvm-svn: 302840